Gas analyzer apparatus

ABSTRACT

Accuracy of thermal conductivity gas analysis is enhanced by bubbling a sample gas and a reference gas through a common reservoir to saturate the gases at equal temperature and pressure. The saturated gases are maintained at equal pressures and at a temperature exceeding their dew points until the two gases are conveyed past filaments of a thermal conductivity detector cell.

United States Patent Carswell, Jr. et al.

Sept. 4, 1973 GAS ANALYZER APPARATUS [56] References Cited Inventors:John D. Carswell, Jr., Bernardsville, NITE STATES PATENTS George W-Gorman, D 2,042,646 6/l936 Willenborg 21/212 F. Ga. 2,591,759 4/1952Zaikowsky 73/27 R 3,354, 2 ll I967 W'll" 73 25 X Assignees: Cow-MacInstrument Company, 05 I I mms Incorporated, Madison, N .J by saidCarswell; The United States of Exam.1er Rlchard Que'.sser

Assistant ExammerStephen A. Kreitman Amerlca as represented by the AnomeB Havi Secretary of the Department of y Health, Education and Welfare,Washington, DC. by said Gorman [57] ABSTRACT Accuracy of thermalconductivity gas analysis is en- Ffled 1971 hanced by bubbling a samplegas and a reference gas AppL No.: 168,994 through a common reservoir tosaturate the gases at equal temperature and pressure. The saturatedgases are maintained at equal pressures and at a temperature 1 exceedingtheir dew points until the two gases are con- Field of Search 73/27 R,23, 25; 13 past filaments thermal mnducmny detect 338/34; 23/232 E, 254E, 232 GC 9 Claims, 4 Drawing Figures 50 I7 GLAUSTE SUPPLY I I I I I 44I I I I I I I Patented Sept. 4, 1973 2 Sheets-Sheet 1 llLl-IIII 93 93 misEm muzumwhmm wv m ATTORNEY Patented Sept. 4, 1973 3,756,069

2 Sheets-Sheet 2 FIG. 2

GAS ANALYZER APPARATUS BACKGROUND OF THE INVENTION paratus includes apair of conduits, or tubes, for conveying sample and reference gases todifferent chambers of a thermal conductivity cell, or detector. Thesample gas includes a carrier gas and an unknown quantity of a solutegas. The reference gas often is the same type of gas as the carrier gasof the sample.

Determination of the quantity of the solute gas is accomplishedaccurately by processing the sample and reference gases at equal flow,pressure, and temperature.

The thermal conductivity cell, or detector, of the apparatus is ametallic block having a pair of gas flow passages for conveying,respectively, the sample and reference gases to detector elements. Twoof the detector elements, or hot wire filaments, are positioned alongeach of the passages so that the elements are exposed either directly orindirectly to the flowing gas. The filaments of the detector arefabricated from material having a high resistivity and a hightemperature coefficient of resistance.

The four filaments of the detector are connected in an electrical bridgecircuit in which the two filaments exposed to each gas are positioned onopposite sides of the bridge. A direct current power supply is appliedacross one diagonal of the bridge for establishing a quiescent currentthrough the four filaments to heat them to an initial temperature abovethe gas temperature.

Loss of heat from each filament depends upon the thermal conductivity ofthe gas surrounding the filament. As the composition of the surroundinggas varies, the thermal conductivity thereof also varies. Thus changesin the composition of the gas cause heat loss from the filament tofluctuate.

Fluctuations of the heat loss cause the temperature of the filaments tovary over a small range. Because of the high temperature coefficient ofresistance in the filaments, the temperature fluctuations cansubstantially change the resistance of the filaments. Such changes ofresistance cause the current conducted through the filaments tofluctuate from the value of the quiescent current.

These fluctuations of current unbalance the bridge and can be detectedby a voltmeter connected across the bridge on a diagonal opposite to thediagonal of the power supply. This meter has a face calibrated to showthe quantity of solute in the sample gas being analyzed. As thefluctuations of the current change the meter reading, the quantity ofsolute gas is read directly from the face of the meter.

This method of measuring the quantity of solute gas in the sample isvery sensitive to differences between the rates of flow, thetemperatures, the pressures, and the moisture contents of the sample andreference gases. If such differences occur, they are interpreted by thedetector and the meter as changes in the quantity of solute gas in thesample.

Various methods have been used to reduce the possibility of differencesof flow, temperature, pressure, and moisture content of the two gases.

One method, used for reducing differences in the moisture contents ofthe sample and reference gases, is accomplished by drying each of thegases in a separate moisture trap. However, it is known that such trapsmay dry the two gases unequally. Such unequal drying causes erroneousquantity readings.

Another method, used for reducing differences in the I moisture content,is accomplished by saturating each of the gases in separate saturators,however, such separate saturators can collect different amounts ofcondensation from the two gases. After collecting condensation for awhile, the columns of water rise to different heights in the respectivesaturators. Such different height columns subject the two gases tounequal pressures. These different pressures also cause erroneousquantity readings.

If saturated gas passes through a flowmeter used for determining andcontrolling the rate of flow, moisture can condense within the flowmetercausing it to malfunction and produce erroneous readings.

Additionally moisture from saturated gas can condense within tubing usedfor conveying the gas from the saturator to the detector. Anydifferential condensation can cause erroneous quantity readings.

Therefore a problem exists in conveying the sample and reference gasesto the thermal conductivity detector with equal rate of flow,temperature, pressure, and moisture content so that inequalities ofthose characteristics do not affect the determination of the quantity ofthe solute gas in the sample.

SUMMARY OF THE INVENTION Therefore it is an object of this invention todevelop an improved thermal conductivity gas analyzer.

It is a further object to improve the accuracy of thermal conductivitygas analyzers.

These and other objects of the invention are realized in a thermalconductivity gas analyzer which conveys sample and reference gasesthrough a common reservoir for saturating the gases equally at the sametemperature and pressure. The saturated gases are conveyed to thefilaments in a thermal conductivity detector cell while being maintainedat equal pressures and at a temperature exceeding their dew points.

It is a feature of the invention to pass the sample and the referencegases through the common reservoir to saturate the gases equally at thesame pressure and temperature even after the saturator has operated forseveral hours.

It is another feature to maintain the saturated gases at equal pressureand at equal rates of flow until the gases pass through the detectorcell.

It is a further feature to maintain the saturated gases at equaltemperatures above their dew points until the gases pass through thedetector cell.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of theinvention may be derived from the detailed description following if thatdescription is considered with respect to the attached drawings wherein:

FIG. 1 is a schematic diagram of a thermal conductivity gas analysisapparatus in accordance with the invention;

FIG. 2 is an illustration of a flowmeter arranged with a heating unit;

FIG. 3 is an illustration of a common reservoir saturator used in theapparatus of FIG. 1; and

FIG. 4 is a schematic diagram of a bridge circuit used in the gasanalysis apparatus.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown a schematicdiagram of a gas analyzer for determining the quan- --tity of a solutegas dissolved in a solvent gas within a housing, or chamber 12. Acontrol system 15, which is responsive to the quantity of soluteregulates the flow of solute gas from a supply 17 to the housing 12. Thesolvent gas, stored in a reference gas supply 18, is carried to thehousing 12 by any suitable means, such as the tubing 19.

In actual practice, the reference gas supply 18 and the method ofcarrying the solvent gas to the housing 12 may vary widely. Forinstance,-the solvent gas often is room air which surrounds the entirehousing 12. The housing 12 is constructed so that air flows into thechamber making the atmosphere within the chamber mostly room air.

The solvent gas is also supplied, as a reference gas, to the gasanalyzer 10 by way of a tube 21. A pump 22 forces the reference gas, orair, to flow along tube 23 into a flowmeter 24.

A sample of the gas mixture contained in the housing 12 can be suppliedto the gas analyzer 10 by way of a tube 26. In the analyzer, a shut offvalve 27 and a tube 28 carry the sample gas to another pump 29, whichforces the sample gas to flow through a tube 31 into another flowmeter32. The sample gas may be a gas such as carbon dioxide having a thermalconductivity nearly equal to the thermal conductivity of the referencegas which may be air. v

The pumps 22 and 29 advantageously may be vibrator type diaphragm pumpsproviding a constant flow rate of approximately 200 cubic centimeters ofgas per minute.

The flowmeters 24 and 32 advantageously are tapered tube flowmeters,each having a metering valve at its gas inlet and each being heatedabove the temperature inside of the housing 12. The metering valves areadjusted so that equal volumes of gas flow through the two flowmeters.

As shown in FIG. 2, the flowmeter 24 has a controllable electric heaterelement 30 inserted inside of its housing. Such a heater element ismechanically attached to the housing of each of the flowmeters 24 and 32in FIG. 1 for maintaining them at a high ambient temperatureso thatmoisture contained in the sample gas and the reference gas will notcondense within the flowmeters 24 and 32. Any condensation in theflowmeters can adversely affect the accuracy of flow readmgs.

Gas outlets of the flowmeters 24 and 32 are piped by way of tubes 33 and34 into a saturator 37 for saturating the sample gas and the referencegas at the same temperature and the same pressure.

A more detailed view of the saturator 37 is shown in FIG. 3, where theinput tubes 33 and 34 are shown entering at the top and bending downtoward the bottom of the saturator.

The saturator 37 includes a common reservoir 38 which is filled withsome fluid, such as water. Placed inside of the reservoir are twocylindrical saturator columns 39 and 40 that are sealed to the top ofthe saturator and to the bottom of the reservoir 38. Bleeder holes 41and 42, located near the bottoms of the columns 39 and 40, enable thefluid to fill the columns equally at all times.

Gases, entering the saturator 37 by way of tubes 33 and 34, are emittedfrom the bottom ends of those tubes and bubble to the surface. Slottedcollector tubes 43 and 44 located above the fluid surface collect thegases which accumulate in the space above the fluid surface in thecolumns 39 and 40'. The gases thus collected in the spaces at the top ofthe separate columns are carried away through the collector tubes 43 and44, shown in FIG. 1, to a gas thermal conductivity cell 46. The ends ofthe collector tubes 43 and 44 in the top of the columns 39 and 40 may beany one of a number of shapes as long as each one of the collector tubescollects all of the gas bubbled in from a separate one of the emittertubes 33 and 34.

As a result of the gases bubbling up through the water in the commonreservoir 38, each gas is saturated equally with the water. The gasesare held at equal temperature and pressure because the gases are exposedto the same reservoir of water. Even though the analyzer 10 is allowedto run unattended for periods as long as 24 hours or more, thetemperature and pressure on each of the gases remains equal. Anycondensation from the gases or evaporation from the reservoir changesthe water level of the entire reservoir and therefore affects thereference and sample gases equally.

If metallic tubing is used for tubes 33, 34, 43, or 44, it isadvantageous to heat those tubes to a uniform temperature above the dewpoint of the gases. This assures that the gases flow at uniformtemperature from the flow-meters 24 and 32 through the saturator 37 tothe detector 46. Electric heating tape 45 attached to the tubes 33, 34,43 and 44 maintains a uniform temperature in those tubes.

Thermal conductivity cell 46 includes a first chamber 47 for carryingthe reference gas to two hot wire filaments R and R and a second chamber48 for carrying the sample gas to two additional hot wire filaments S 1and 8 All of the filaments R R 8,, and S, may advantageously be rheniumtungsten filaments that are inserted into the chambers so that thereference and sample gases are exposed to the filaments by diffusionrather than by direct flow.

The filaments R R 5,, and S are connected in an electrical bridgecircuit, as shown in FIG. 4. A battery 50 supplies operating power tothe bridge arrangement. Adjustable slidewire resistors are includedwithin the bridge for balancing the bridge. A voltmeter is shownconnected across one diagonal of the bridge between nodes 56 and 57.When no current flows in that branch, the voltmeter 55 reads zero.

Before the apparatus is used for determining the quantity of solute gasin the sample obtained from the housing 12 in FIG. 1, it is essential tocalibrate the voltmeter 55. Such calibration is accomplished in thefollowing manner.

Valves 27 and 51 are closed, and valve 52 is opened to accomplish thefirst step of calibration. The reference gas is then conveyed throughboth paths to the flowmeters 24 and 32. The flow of reference gas isequalized in both paths by means of the metering valves in theflowmeters. Both streams of reference gas are bubbled through thesaturator 37 and are conveyed to the separate chambers of the cell 46.

In the thermal conductivity cell 46, the filaments are exposed toidentical gases having identical thermal conductivity. Slidewire 58, inFIG. 4, is adjusted so that no current flows through the branch betweennodes 56 and 57 and so that the bridge operates in its quiescentcondition. The voltmeter 55 reads zero indicating that there is nosolute gas conveyed past elements S, and S Once the apparatus thus isadjusted for zero, a span gas and the reference gas are conveyed to theanalyzer 10. Span gas is a solvent gas that contains a known quantity ofthe solute gas to be mixed in the atmosphere within the housing 12. Thespan gas from the supply 60, in FIG. 1, is conveyed to the analyzer byopening valve 51 so that the span gas flows through tube 28, pump 29,and tube 31 to the flowmeter 32. By closing valve 52 the reference gasflows only through tube 21, pump 22, and tube 23 to the flowmeter 24.Valve 27 remains closed.

The span gas and the reference gas thus entering the analyzer l0 flow byway of separate paths through the flowmeters and the saturator to thecell 46.

Once the span gas and the reference gas are conveyed to the cell 46, thethermal balance of the filaments will change and cause the bridge tounbalance. Such unbalance makes the voltmeter 55 deflect from zero. Atthe point of deflection, the face of the meter is marked with a valueequal to the known per cent of solute in the span gas. Thus the zero andan upscale reading are accurately determined on the face of thevoltmeter 55 completing the calibration of the meter.

Full scale meter deflection usually is limited to approximately 10 percent solute gas in the solvent gas. The scale can be divided into equalunits because changes of thermal conductivity are linear over the rangeof solute gas used.

After the meter is calibrated by the reference gas and span gas, asample of gas from the housing 12 can be analyzed by concurrentlyconveying the reference gas from the supply 18 and the sample gas fromthe housing 12 to the cell 46. This is accomplished by closing the valve51 and opening the valve 27. Valve 52 remains closed. The sample gas isconveyed through the valve 27, the pump 29, and the flow-meter 32 to thesaturator 37. The reference gas is conveyed through the pump 22 and theflowmeter 24 to the saturator 37. Both gases are bubbled through thecommon reservoir 38 and are conveyed by way of the separate collectortubes 43 and 44 to the detector cell 46 for analysis.

Because of differences in the thermal conductivity of the sample gas andthe reference gas caused by the quantity of solute in the sample gas,the temperature of filaments S, and S fluctuate with respect to thetemperature of the filaments R, and R This fluctuation of temperaturechanges the resistance of the filaments S, and S with respect to theresistance of the filaments R, and R and the bridge is unbalanced. As aresult the voltmeter 55 deflects a distance dependent upon the quantityof solute gas in the sample gas.

As shown in FIG. 4, the sample gas filaments S, and S are located inopposite branches of the bridge. The reference gas filaments R, and Rare also placed in opposite branches of the bridge. Chambers 47 and 48of the detector cell 46 are indicated in FIG. 4 by dashed lines andarrows showing the direction of gas flow therethrough.

This arrangement for exposing the sample and reference gases to thefilaments causes the filaments to change temperature in response tovariations in the thermal conductivity of the gases, as previouslydiscussed. In particular, the sample filaments increase in temperatureas the amount of carbon dioxide in the sample gas increases because thethermal conductivity of the carbon dioxide is less than the thermalconductivity of air. The positive temperature coefficient of resistanceof the filament material causes the resistance of the sample filamentsS, and S to increase with increasing temperature.

Such an increase in the resistance of the sample filaments with respectto the reference filaments causes current flow in the meter branch ofthe bridge. This current is manifested as a voltage drop between nodes56 and S7 in the bridge. This voltage drop is measured by the voltmeter55.

The parts of the gas analyzer described previously comprise the basicgas analysis apparatus. In FIG. 1, additional apparatus is included forcontrolling the amount of solute gas that is mixed with the referencegas in the housing 12.

Thus in FIG. 1 the meter movement of voltmeter 55 is arranged to operatea pair of limit switches 61 and 62 in the control system 15. Theswitches 61 and 62, respectively, control slow flow and fast flowsolenoid valves 63 and 64.

The control system 15 operates so that an amount of solute gas can beadded rapidly to the housing 12 and so that the amount of solute gas inthe housing can be maintained automatically between predetermined lowerand upper limits.

Operation of the fast flow valve 64 enables the chamher to fill rapidlywith solute gas when the solute gas is first added to the housing 12 orwhen the amount of solute gas in the housing 12 falls below thepredetermined lower limit. Thus the limit switch 62 is set to operatethe solenoid and open the fast flow valve 64 when the amount of solutegas in the housing is less than the predetermined lower limit. The limitswitch 62 also is set to close the fast flow valve 64 when the amount ofsolute gas in the housing 12 is greater than the predetermined lowerlimit.

Operation of the slow flow valve 63 maintains the amount of solute gasinside of the housing 12 within the predetermined lower and upperlimits. Thus the limit switch 61 is set to operate the solenoid and openthe slow flow valve 63 when the amount of solute gas in the housing 12is less than half way between the lower and upper limits. Switch 61 alsois set to disable the solenoid and close the slow flow valve 63 when theamount of solute gas inside of the housing is greater than half waybetween the lower and upper limits.

Separate branches of tubing connect the solute gas supply 17 to thevalves 63 and 64 and thence to the housing 12 for supplying solute gasto the housing whenever one or both of the valves 63 and 64 are open.Thus when the meter 55 indicates that the per cent of solute gas hasdecreased toward or below the predetermined lower limit, additionalsolute gas is supplied from the supply 17 through one or both of thevalves 63 and 64 to the housing 12. Thus additional solute gas issupplied to the housing 12 until the per cent of solute gas in thehousing 12 has increased somewhat above the lower limit. Then the solutegas from the supply 17 is cut off by the valves 63 and 64.

The foregoing discussion describes a control system operating inresponse to the gas analyzer 10, which is arranged to accuratelydetermine the quantity of solute gas in the solvent gas by assuring thatsample and reference gases are equally saturated at equal temperaturesand pressures and are held above their dew points until after the gasespass the filaments in the detector cell 46.

The above detailed description is illustrative of an embodiment of theinvention, and it is understood that additional embodiments thereof willbe obvious to those skilled in the art. All of these embodiments areconsidered to be within the scope of the invention.

What is claimed is:

1. A gas mixture analyzer comprising a thermal conductivity detectorcell having first and second gas chambers, each chamber enclosing a pairof electrical conductors,

means for conveying a first gas to the conductors in the first chamber,

means for conveying a second gas to the conductors in the secondchamber,

means for heating the gas conveying means, and

a common reservoir interposed in the first and second gas conveyingmeans for saturating the first and second gases at equal temperature andequal pressure.

2. A gas analyzer in accordance with claim 1, wherein the means forconveying the first and second gases each comprise means for determiningthe rate of gas flow, and

the heating means includes means for heating the rate of flowdetermining means to a temperature above the dew point of the gases.

3. A gas analyzer in accordance with claim I wherein the commonreservoir comprises:

means for containing a fluid,

first and second collectors,

means for bubbling the first gas through the fluid to the firstcollector, and

means for bubbling the second gas through the fluid to the secondcollector.

4. A gas analyzer in accordance with claim 3 wherein the means forconveying the first and second gases each comprises means fordetermining the rate of gas flow therethrough, and

the heating means include means for heating the rate of flow determiningmeans to a temperature above the dew point of the gases. 5. A gasanalyzer in accordance with claim 4 wherein the conductors, enclosed inthe first and second chambers, interconnect in an electrical bridgecircuit for 5 controlling a meter in response to changes of compositionof the second gas with respect to the first gas.

6. A thermal conductivity gas analyzer comprising first and secondfilaments in a first chamber,

third and fourth filaments in a second chamber,

means for connecting the first, second, third, and

fourth filaments in a bridge circuit,

a gas saturator including a common reservoir having a plurality of pathsfor saturating gases,

means for conveying a first gas at a predetermined rate through one pathof the saturator to the first chamber,

means for conveying a second gas at the predetermined rate throughanother path of the saturator to the second chamber, and

means interposed along both conveying means for heating the first andsecond gases above their dew points.

7. A gas analyzer in accordance with claim 6 wherein the means forconveying gases each includes:

means for emitting the gas into an enclosed column of fluid within thereservoir,

means for collecting all of the gas emitted within the column, and

fluid means for filling the common reservoir and the columns to auniform level.

8. A gas analyzer in accordance with claim 7 wherein the conveying meanseach comprises means for determining the rate of flow of gas, and

the heating means are attached to the rate of flow determining means forheating the gases above their dew points.

9. A gas mixture analyzer comprising a thermal conductivity detectorcell having first and second gas chambers, each chamber enclosing a pairof electrical conductors,

means for conveying a first gas to the conductors in the first chamber,

means for conveying a second gas to the conductors in the secondchamber, and

a common reservoir interposed in the first and second gas conveyingmeans for saturating the first and second gases at equal temperature andequal pressure.

1. A gas mixture analyzer comprising a thermal conductivity detectorcell having first and second gas chambers, each chamber enclosing a pairof electrical conductors, means for conveying a first gas to theconductors in the first chamber, means for conveying a second gas to theconductors in the second chamber, means for heating the gas conveyingmeans, and a common reservoir interposed in the first and second gasconveying means for saturating the first and second gases at equaltemperature and equal pressure.
 2. A gas analyzer in accordance withclaim 1, wherein the means for conveying the first and second gases eachcomprise means for determining the rate of gas flow, and the heatingmeans includes means for heating the rate of flow determining means to atemperature above the dew point of the gases.
 3. A gas analyzer inaccordance with claim 1 wherein the common reservoir comprises: meansfor containing a fluid, first and second collectors, means for bubblingthe first gas through the fluid to the first collector, and means forbubbling the second gas through the fluid to the second collector.
 4. Agas analyzer in accordance with claim 3 wherein the means for conveyingthe first and second gases each comprises means for determining the rateof gas flow therethrough, and the heating means include means forheating the rate of flow determining means to a temperature above thedew point of the gases.
 5. A gas analyzer in accordance with claim 4wherein the conductors, enclosed in the first and second chambers,interconnect in an electrical bridge circuit for controlling a meter inresponse to changes of composition of the second gas with respect to thefirst gas.
 6. A thermal conductivity gas analyzer comprising first andsecond filaments in a first chamber, third and fourth filaments in asecond chamber, means for connecting the first, second, third, andfourth filaments in a bridge circuit, a gas saturator including a commonreservoir having a pLurality of paths for saturating gases, means forconveying a first gas at a predetermined rate through one path of thesaturator to the first chamber, means for conveying a second gas at thepredetermined rate through another path of the saturator to the secondchamber, and means interposed along both conveying means for heating thefirst and second gases above their dew points.
 7. A gas analyzer inaccordance with claim 6 wherein the means for conveying gases eachincludes: means for emitting the gas into an enclosed column of fluidwithin the reservoir, means for collecting all of the gas emitted withinthe column, and fluid means for filling the common reservoir and thecolumns to a uniform level.
 8. A gas analyzer in accordance with claim 7wherein the conveying means each comprises means for determining therate of flow of gas, and the heating means are attached to the rate offlow determining means for heating the gases above their dew points. 9.A gas mixture analyzer comprising a thermal conductivity detector cellhaving first and second gas chambers, each chamber enclosing a pair ofelectrical conductors, means for conveying a first gas to the conductorsin the first chamber, means for conveying a second gas to the conductorsin the second chamber, and a common reservoir interposed in the firstand second gas conveying means for saturating the first and second gasesat equal temperature and equal pressure.